Amethyst is one ofthe most secure Nostr apps in the market. nostr:nprofile1qqs24yz8xftq8kkdf7q5yzf4v7tn2ek78v0zp2y427mj3sa7f34ggjcpzamhxue69uhhv6t5daezumn0wd68yvfwvdhk6tcppemhxue69uhkummn9ekx7mp0qyg8wumn8ghj7mn0wd68ytnddakj703s8dt team has gone above and beyond in privacy and security. Supports Amber, TOR natively (optional), comprehensive relay control, filter control (the user is empowered to decide what shows in his/her feed), encrypted messages and off course, open source.

The fact that is not available for Apple OS ia a completely different matter.

That being said, perhaps has to do with focusing resources according to market share of the OS, iOS is not that popular across the world:

Data up to date Q1 2025D

Hence, if the intention was to use the resources wisely to reach and get into NOSTR the most possible people across the world, Android seems the most logical choice.